It is conventionally known to use a foil bearing as a bearing for a shaft (or journal) that rotates at a high speed such as at tens of thousands rpm, in which the foil bearing comprises a plurality of foils (flexible membranes) for forming a bearing surface and supports the shaft by means of pressure of a fluid dragged in between the shaft and the foils as the shaft rotates. In some of such foil bearings, a foil assembly is constituted by a cylindrical bearing sheet and a plurality of bump foils, each of which consists of a corrugated thin plate and is disposed radially outward of the cylindrical bearing sheet (see U.S. Pat. No. 4,277,113, for example).
In the foil bearings of this type, it is known that a resonance phenomenon may occur where an oscillation amplitude sharply increases at a particular rotation speed determined by the mass of the rotating member and the support rigidity of the bump foils. In order to reduce the oscillation amplitude at the resonance phenomenon, it is desirable to minimize the dynamic unbalance of the rotating member as well as to maximize the support rigidity of the foil assembly (or frictional damping force that the foil assembly generates against a displacement of the shaft).
As shown in FIG. 5, however, a conventional corrugated bump foil F contacts radially adjoining members (a stationary mount member 3a and a top foil 5a) with its curved surfaces and thus circumferential dimensions A, B of contact areas between the bump foil F and the top foil 5a and between the bump foil F and the stationary mount member 3a are quite limited, and this makes it difficult to achieve an adequate frictional damping force.